Digital bone densitometry devices such as the DPX machines manufactured by LUNAR Corporation of Madison, Wis. or the QDR machines manufactured by Hologic, Inc. of Waltham, Mass., are used to generate broadly based values of bone character, such as bone mineral content ("BMC") or bone mineral density ("BMD"). Such information about bone character, and in particular, about bone character in the spine is often relied on to diagnose and treat bone depletive disorders such as osteoporosis. In the case of osteoporosis, bone density measurements alone are not definitive for diagnosis. The clinician must also look for evidence of spinal fracture. J. A. Kanis, et al. Osteoporosis Int. 1:182-188 (1991) Determining whether a fracture is present is important both on clinical grounds and for research purposes. In the clinical setting, a patient may display a reduced BMD but the clinician is hesitant or unwilling to begin a particular treatment regimen without a diagnosis of fracture or deformity. In the research setting, diagnosis of fracture is important in studying the incidence and prevalence of osteoporosis in a population, or as an entry criterion to a clinical study, or as a measure of efficacy with regard to a particular treatment. In this regard, the European Foundation for Osteoporosis has published guidelines for clinical trials in osteoporosis which recommends a definition of osteoporosis as a "disorder where one or more fractures has arisen due to an increase in the fragility of bone," and an endpoint of fracture reduction in studies of efficacy of new drugs for the treatment of osteoporosis. J. A. Kanis, et al.
While the presence or absence of vertebral fracture is critical in the diagnosis of osteoporosis, diagnosis of vertebral fracture is often difficult. Over one-half of such fractures are asymptomatic, and in cases of minimal symptoms obvious fracture or deformity will often not be observed particularly if there is no previous radiological record for comparison.
Vertebral morphometry techniques promise to make the determination of vertebral fracture or deformation more objective. These approaches rely on certain indexes or normative values of vertebral body dimensions. See e.g. Minne et al., "A Newly Developed Spine Deformity Index (SDI) to Quantitate Vertebral Crush Factors in Patients with Osteoporosis," Bone and Mineral, 3:335-349 (1988); J. C. Gallagher et al., "Vertebral Morphometry: Normative Data," Bone and Mineral, 4:189-196 (1988); Hedlund et al., "Vertebral Morphometry in Diagnosis of Spinal Fractures," Bone and Mineral, 5:59-67 (1988); and Hedlund et al., "Change in Vertebral Shape in Spinal Osteoporosis," Calcified Tissue International, 44:168-172 (1989).
In using vertebral morphometry to diagnose fractures, the clinician commonly employs analog radiological imaging techniques. In essence, an analog x-ray image of the patient's vertebrae is taken, and printed onto a fixed media, such as an x-ray radiographic film print. The print is made to a specific scale relative to the original human, i.e., one-to-one, or a specifically reduced or expanded scale. Then the clinician manually measures the size of a vertebra by using a ruler and a straight edge and actually draws on the film to outline the vertebral body, and then measures with the ruler between criteria lines drawn onto the film itself.
There have been recent efforts to computerize this morphometric technique. See e.g. Nelson, et al., "Measurement of Vertebral Area on Spine X-rays in Osteoporosis: Reliability of Digitizing Techniques", J. Bone add Mineral Res., Vol. 5, No. 7:707-716 (1990); Smith-Bindman et al., "The index of radiographic area (IRA): a new approach to estimating the severity of vertebral deformity", Bone and Mineral, 15:137-150 (1991). These efforts still rely on first obtaining an analog x-ray image of the vertebra, digitizing the analog image and then manually selecting the points of measurement.
Thus the clinician diagnosing or treating osteoporosis must, at a minimum, use two relatively expensive medical devices: a bone densitometer and an x-ray imaging device. Further, morphometric techniques which rely on analog radiography are complicated by image magnification. The analog radiographic image is typically 10-15% larger than life-size, and the magnification is variable depending on the location of the object relative to the plane of the radiograph. In particular, the front edge of the object, away from the radiographic plate is more magnified than the back edge toward the radiographic plate. The result is that bone edges perpendicular to the plane of the plate, which for morphological measurement should produce a sharp visual demarcation on the fan beam radiograph produce a blurred boundary. Distortions of the spine are particularly acute for cone beam exposures at the edges of the cone beam where the beam is most angled. For vertebral morphology the angulation obscures and distorts intervertebral spacing at the top and bottom of a field rendering morphological measurements, for example of body height, imprecise. This impreciseness is exacerbated by the imposition of human error when it is left to the clinician to manually select the measurement point. Variation will often necessarily exist between clinicians and between measurements by the same clinician at different times. These problems apply, not just to the detection of osteoporosis, but to morphometric measurements of bone in general.
While bone densitometers such as the DPX and the QDR devices are capable of generating images, the image quality of these present day bone densitometers is inferior to the common analog x-ray imaging machine. This is particularly true for scanning systems where resolution is intentionally limited to prevent the need for an overly long scanning time. Thus, the imaging capability of bone densitometers has not been relied on for diagnostic purposes and until the present invention, bone densitometry systems have not been used to determine bone morphology, or to analyze the relationships of bone structures.